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Message-ID: <CACLfguWWcTNRXt7KO4D-Bwno2QF14Tkptw3HwM3P_RX62zu=uw@mail.gmail.com>
Date: Thu, 5 Jun 2025 15:34:01 +0800
From: Cindy Lu <lulu@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: jasowang@...hat.com, michael.christie@...cle.com, sgarzare@...hat.com,
linux-kernel@...r.kernel.org, virtualization@...ts.linux-foundation.org,
netdev@...r.kernel.org
Subject: Re: [PATCH RESEND v10 3/3] vhost: Add new UAPI to select kthread mode
and KConfig to enable this IOCTL
On Sun, Jun 1, 2025 at 6:49 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Sat, May 31, 2025 at 05:57:28PM +0800, Cindy Lu wrote:
> > This patch introduces a new UAPI that allows the vhost device to select
> > in kthread mode. Userspace applications can utilize IOCTL
> > VHOST_FORK_FROM_OWNER to select between task and kthread modes, which
> > must be invoked before IOCTL VHOST_SET_OWNER, as the worker will be
> > created during that call.
> >
> > The VHOST_NEW_WORKER requires the inherit_owner setting to be true, and
> > a check has been added to ensure proper configuration.
> >
> > Additionally, a new KConfig option, CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL,
> > is introduced to control the availability of the IOCTL
> > VHOST_FORK_FROM_OWNER. When CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL is set
> > to n, the IOCTL is disabled, and any attempt to use it will result in a
> > failure.
> >
> > Signed-off-by: Cindy Lu <lulu@...hat.com>
>
> I propose renaming
> CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL
> to
> CONFIG_VHOST_ENABLE_FORK_OWNER_CONTROL
> and it should also control the presence of the module parameter
> and a get ioctl (more on which below).
>
> Otherwise we can get a situation where task mode is disabled and
> there is no way for userspace to override or check.
>
>
sure, will do
Thanks
cindy
>
> > ---
> > drivers/vhost/Kconfig | 13 +++++++++++++
> > drivers/vhost/vhost.c | 30 +++++++++++++++++++++++++++++-
> > include/uapi/linux/vhost.h | 16 ++++++++++++++++
> > 3 files changed, 58 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
> > index 020d4fbb947c..300e474b60fd 100644
> > --- a/drivers/vhost/Kconfig
> > +++ b/drivers/vhost/Kconfig
> > @@ -96,3 +96,16 @@ config VHOST_CROSS_ENDIAN_LEGACY
> > If unsure, say "N".
> >
> > endif
> > +
> > +config VHOST_ENABLE_FORK_OWNER_IOCTL
> > + bool "Enable IOCTL VHOST_FORK_FROM_OWNER"
> > + default n
> > + help
> > + This option enables the IOCTL VHOST_FORK_FROM_OWNER, allowing
> > + userspace applications to modify the thread mode for vhost devices.
> > +
> > + By default, `CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL` is set to `n`,
> > + which disables the IOCTL. When enabled (y), the IOCTL allows
> > + users to set the mode as needed.
> > +
> > + If unsure, say "N".
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 2d2909be1bb2..cfa60dc438f9 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -1022,6 +1022,13 @@ long vhost_worker_ioctl(struct vhost_dev *dev, unsigned int ioctl,
> > switch (ioctl) {
> > /* dev worker ioctls */
> > case VHOST_NEW_WORKER:
> > + /*
> > + * vhost_tasks will account for worker threads under the parent's
> > + * NPROC value but kthreads do not. To avoid userspace overflowing
> > + * the system with worker threads inherit_owner must be true.
> > + */
> > + if (!dev->inherit_owner)
> > + return -EFAULT;
> > ret = vhost_new_worker(dev, &state);
> > if (!ret && copy_to_user(argp, &state, sizeof(state)))
> > ret = -EFAULT;
> > @@ -1138,7 +1145,7 @@ void vhost_dev_reset_owner(struct vhost_dev *dev, struct vhost_iotlb *umem)
> > int i;
> >
> > vhost_dev_cleanup(dev);
> > -
> > + dev->inherit_owner = inherit_owner_default;
> > dev->umem = umem;
> > /* We don't need VQ locks below since vhost_dev_cleanup makes sure
> > * VQs aren't running.
> > @@ -2292,6 +2299,27 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
> > goto done;
> > }
> >
> > +#ifdef CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL
> > + if (ioctl == VHOST_FORK_FROM_OWNER) {
> > + u8 inherit_owner;
> > + /*inherit_owner can only be modified before owner is set*/
> > + if (vhost_dev_has_owner(d)) {
> > + r = -EBUSY;
> > + goto done;
> > + }
> > + if (copy_from_user(&inherit_owner, argp, sizeof(u8))) {
> > + r = -EFAULT;
> > + goto done;
> > + }
> > + if (inherit_owner > 1) {
> > + r = -EINVAL;
> > + goto done;
> > + }
> > + d->inherit_owner = (bool)inherit_owner;
> > + r = 0;
> > + goto done;
> > + }
> > +#endif
> > /* You must be the owner to do anything else */
> > r = vhost_dev_check_owner(d);
> > if (r)
> > diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> > index d4b3e2ae1314..d2692c7ef450 100644
> > --- a/include/uapi/linux/vhost.h
> > +++ b/include/uapi/linux/vhost.h
> > @@ -235,4 +235,20 @@
> > */
> > #define VHOST_VDPA_GET_VRING_SIZE _IOWR(VHOST_VIRTIO, 0x82, \
> > struct vhost_vring_state)
> > +
> > +/**
> > + * VHOST_FORK_FROM_OWNER - Set the inherit_owner flag for the vhost device,
> > + * This ioctl must called before VHOST_SET_OWNER.
> > + *
> > + * @param inherit_owner: An 8-bit value that determines the vhost thread mode
> > + *
> > + * When inherit_owner is set to 1(default value):
> > + * - Vhost will create tasks similar to processes forked from the owner,
> > + * inheriting all of the owner's attributes.
> > + *
> > + * When inherit_owner is set to 0:
> > + * - Vhost will create tasks as kernel thread.
> > + */
> > +#define VHOST_FORK_FROM_OWNER _IOW(VHOST_VIRTIO, 0x83, __u8)
>
>
>
> Given default now depends on the module parameter, we should
> have both GET and SET ioctls. All controlled by the kconfig knob.
>
Sure, will do
Thanks
cindy
> > +
> > #endif
> > --
> > 2.45.0
>
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